a) Field of the Invention
This invention relates to dispersants for organic pigments, readily-dispersing pigment compositions, pigment dispersions, colored coating fluids for color filters (colored coating fluids, which are colored with pigments, will hereinafter be called simply “coating fluids”), a process for the fabrication of color filters, and color filters.
b) Description of the Related Art
A color filter for use in a liquid crystal display or the like is fabricated primarily by a so-called pigment dispersion process (i.e., a process making use of a pigment as a colorant), specifically by applying a coating fluid, which has been prepared by dispersing a pigment in a photoresist (a solution of a photosensitive resin), to a substrate for the color filter by a coating method such as spin coating or by electrodeposition, exposing the thus-formed, colored coating film to radiation, and then developing and patternizing the coating film to form pixels, and repeating the above-mentioned process until RGB (red, green and blue) pixels are formed in predetermined patterns.
As a coloring pigment for a coating fluid to be used for the formation of blue pixels upon fabrication of the color filter, dioxazine violet (C.I. Pigment Violet 23) has been employed for the purpose of a color correction (toning) in addition to ε-phthalocyanine blue (C.I. Pigment Blue 15:6). Especially to provide blue pixels of color filters for TV monitors, the demand for which has been considerably increasing in recent years, with more preferred color quality, it is required to increase the proportion of dioxazine violet relative to ε-phthalocyanine blue.
Upon dispersing the mixed pigment, which consists of ε-phthalocyanine blue and dioxazine violet in a dispersion medium such as a photoresist, simple dispersion by a conventional disperser is unable to achieve full dispersion of the mixed pigment. If blue pixels are formed from a coating fluid so obtained, the pixels lack of light permeability, and as blue pixels of a color filter, are insufficient in light transmittance. The coating fluid with the above-described mixed pigment contained therein is, therefore, insufficient as a coating fluid for color filters.
As a resin commonly used in a photoresist as a dispersion medium for the above-described mixed pigment, on the other hand, a high acid value acrylic polymer is principally employed to permit the development of a colored coating film with an aqueous alkaline solution after exposure. With the coating fluid formed of the mixed pigment and the photoresist which contains the high acid value acrylic polymer, however, there are problems in that the pigment is prone to flocculation so that the viscosity of the coating fluid tends to become higher and also in that with time, the coating fluid becomes more viscous and deteriorates in storage stability.
To form pixels of each color for a color filter with a coating fluid which involves problems as described above, the coating fluid is applied to a substrate by spin coating, and the resulting coating film is then exposed and developed for its patternization. If the coating fluid so employed has high viscosity or it shows thixotropic viscosity due to flocculation of the pigment, the coating film formed of the coating fluid has a rise around an central part thereof (before its exposure). When a color filter is fabricated for use in a large-screen LCD, there is a problem in that unevenness and/or a difference arises in hue between pixels at a central part of the substrate and those at its peripheral part.
A coating fluid for color filters generally contains a pigment in a high concentration range of from 5 to 20 wt. %. Nonetheless, the coating fluid is required to have such a state of dispersion that its pigment particles do not undergo flocculation, it has a lower viscosity than ordinary room-temperature drying coating formulations or baking coating formulations, and it is excellent in storage stability.
To meet the above-described requirements, various methods have been proposed to date, such as adding a substituted derivative of phthalocyanine blue or a substituted derivative of dioxazine violet as a dispersant to the pigment where the pigment is ε-phthalocyanine blue or treating the pigment with the above-described derivative. Reference may be had, for example, to JP-A-56-167762, JP-A-1-34268, JP-A-4-246469, JP-A-6-240161, JP-A-6-240162 and JP-A-7-188576.
From a demand for further improvements in the performance of color filters, on the other hand, it has become increasingly needed to improve the transparency of color pixels, to increase the contrast among lights transmitted through the color pixels, and also to increase the concentrations of pigments in the respective color pixels. With the method that uses the above-described pigment derivative as a dispersant for the dispersion of the pigment, however, it is difficult to provide the color pixels with improved transparency or to avoid an increase in viscosity and a reduction in storage stability, both of which take place due to an increase in the concentration of the pigment, by relying upon an improvement in the dispersibility of the pigment. There is, accordingly, an outstanding desire for improvements in these matters.